The present invention relates to an improved direct-current (DC) arc welding machine, and more particularly, to an improved method of controlling applied to a DC arc welding machine.
A block diagram of a conventional DC arc welder is shown in FIG. 1, wherein the numeral 1 indicates a consumable elctrode (hereunder referred to as a wire), 2 is an arc, 3 is a workpiece (hereunder referred to as the base metal), 4 is a DC power supply including a welding transformer, 5 is a switching element for controlling the welding current supplid from the DC supply to the arc 2, 6 is a drive circuit for driving the switching element 5, 7 is variable resistor for controlling the feed rate of the wire, across which a given voltage is applied, 8 is a motor control circuit for controlling the rotation of a motor according to the value at which the variable resistor 7 is set, and 9 is the wire feeding motor. Reference numeral 10 designates a drive roller driven by the wire feeding motor 9 to feed the wire 1 toward the base metal 3, and 11 is a base current setting circuit for setting the welding current flowing through the arc 2 in accordance with the value to which the variable resistor 7 is set. A shorting detection circuit 12 can detect a short circuit between the wire 1 and the base metal 3, and shorting current setting circuit 13 supplies a large shorting current to the wire in response to a command from the shorting detection circuit 12 and also supplies a large current in a period immediately following the removal of the short circuit. A selector circuit 14 discriminates, in accordance with a command from the shorting detection circuit 12, the value set by the base current setting circuit 11 from the value set by the shorting current setting circuit 13. A current detecting element 15 is provided for detecting the welding current, and a current control circuit 16 controls the output current in accordance with a feedback signal from the current detecting element 15 and a set signal from the selector circuit 14. Numeral 17 indicates a flywheel diode.
In a welding method where the wire 1 is automatically fed toward the base metal 3, the heat input to the base metal 3 is desirably small. To meet this end, the length of the arc 2 (arc length) is reduced. When the wire 1 and base metal 3 are shorted, the molten globule at the tip of the wire 1 is transferred to the base metal 3 to establish an arc between the wire and base, and then the wire and base are shorted to develop an arc. This method of welding, which depends on the cyclic establishment of short circuits and arcs is referred to as short circuit transfer welding. FIG. 1 is thus a block diagram showing an embodiment of a DC arc welding machine which performs short circuit transfer welding.
FIG. 2 is a diagram showing the waveform of the welding current produced by the welding machine of FIG. 1. In FIG. 2, time is plotted on the horizontal axis, and welding current on the vertical axis. In the time period Ts, the wire 1 and base metal 3 are shorted and the welding current is increased. In the time period Ta1 that immediately follows the removal of the short circuit, an arc is developed between the wire 1 and base metal 3 and the welding current is decreased. In the time period Ta2, an arc is also developed between the wire 1 and the base metal 3, but the welding current is at a constant level IB.
In FIG. 1, welding is begun by feeding the wire toward the base metal 3. The wire feed rate is controlled by the number of revolutions of the motor 9, which is turn is controlled by the variable resistor 7. Therefore, the wire feed rate can be controlled by the variable resistor 7. The welding current is controlled by the current control circuit 16, which compares a feedback signal ef from the current detection element 15 with a current set signal ei and delivers two signals, e.g. "high" and "low", to the drive circuti 6. The drive circuit 6 sends a drive signal to the switching element 5 in response to the "high" or "low" signals. When the switching element 5 is conducting, welding current flows from the DC power supply 4 through a path comprising the switching element 5, the wire 1, the arc 2, the base metal 3 and the current detection element 15, and returns to the DC power supply 4. When the switching element 5 is not conducting, a current delay factor (not shown) in the circuit causes the welding current to flow through a path comprising the flywheel diode 17, wire 1, arc 2, base metal 3 and current detection element 15, and returns to the flywheel diode 17, and at no time does the welding current become zero immediately after the switching element 5 is rendered nonconducting. Therefore, a constant current can be supplied to the arc 2 by causing the switching element 5 to alternate between its "conducting" and "nonconducting" states at a given rate and by utilizing feedback control on the welding current.
The function of the selector circuit 14 is to transmit to the current control circuit 16 either a current setting signal e.sub.is from the shorting current setting circuit 13 or a current setting signal e.sub.ia from the base current setting circuit 11 in response to a command from the shorting detection circuit 12. When the wire and base metal are short-circuited, a command from the shorting detection circuit 12 causes the selector circuit 14 to transmit, to the current control circuit 16, the current setting signal e.sub.is from the shorting current setting circuit 13 as the signal ei. The current control circuit 16 compares a feedback signal ef from the current detection element 15 with the signal ei from the selector circuit 14 and delivers a "high" of "low" signal to the drive circuit 6. The drive circuit 6 amplifies the "high" and "low" signals from the current control circuit 16 and turns the switching element 5 on or off in response to the amplified signals.
A molten globule at the tip of the wire is transferred to the base metal and the short circuit is removed. When the short circuit between the wire and the base metal is removed, the short detection circuit 12 delivers a short circuit removal signal e.sub.is to both the shorting current setting circuit 13 and selector circuit 14 for a certain period (the time period Ta1 in FIG. 2). The shorting current setting circuit 13 then outputs a shorting current setting signal e.sub.is that causes the large welding current immediately after the removal of the short circuit to be changed to a relatively small welding current IB without disturbing the arc being developed between the wire and the base metal. While receiving the short circuit removal signal e.sub.is from the shorting detection circuit 12, the selector circuit 14 sends a short current setting signal e.sub.is from the setting circuit 13 to the current control circuit 16 as the signal ei, and as a result, the welding current is changed from the large shorting current to a relatively small welding current IB without disturbing the arc being developed between the wire and the base metal. In the subsequent period (time period Ta2 in FIG. 2) until the wire and the base metal are again short-circuited, the short detection circuit 12 remains off, so the selector circuit 14 cuts off the shorting current setting signal e.sub.is from the shorting current setting circuit 13 and transmits the base current setting signal e.sub.ia from the base current setting circuit 11 to the current control circuit 16 as the signal ei. In consequence, the welding current is maintained at a constant level IB during the period Ta2.
With the circuit configuration described above, the DC arc welder of FIG. 1 is capable of welding with a consumable electrode using a power supply having constant current characteristics. But on the other hand, if the time period during which the arc is developed between the wire and the base metal is decreased, the period Ta2 is correspondingly decreased, and the control of the welding current IB with the variable resistor 7 alone is not capable of producing consistent welding because less wire is melted in comparison with the wire feed rate. Therefore, the defect of the conventional DC arc welder is that it is capable of consistent welding within only a limited range of wire feed rates.